The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the production of angiograms using data acquired with fast NMR pulse sequences.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped, and hence the magnitude of the net transverse magnetic moment M.sub.t depends primarily on the length of time and the magnitude of the applied excitation filed B.sub.1.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. In simple systems the excited spin induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of the transverse magnetic moment M.sub.t.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements." Such NMR measurements are divided into a period of RF excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more RF excitation pulses (B.sub.1) of varying magnitude, duration, and direction. Such excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which produces transverse magnetization M.sub.t over a range of resonant frequencies. The prior art is replete with excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
NMR data for constructing images can be collected using one of many available techniques. Typically, such techniques comprise a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more NMR experiments, each of which comprises at least an RF excitation pulse and a magnetic field gradient pulse to encode spatial information into the resulting NMR signal.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. Other echo-planar pulse sequences are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,716,369; 4,355,282; 4,588,948 and 4,752,735.
Another pulse sequence used to acquire NMR image data quickly is known generally as a fast spin echo (FSE) pulse sequence. One such FSE pulse sequence known as a "Rapid Acquisition Relaxation Enhanced" (RARE) sequence is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled "RARE Imaging: A Fast Imaging Method For Clinical MR." A difference between the RARE sequence and the EPI sequence lies in the manner in which echo signals are produced. The RARE sequence utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill (CPMG) sequence, while EPI methods employ gradient recalled echoes.
Yet another pulse sequence used to acquire NMR image data quickly is known in the art generally as fast gradient-echo pulse sequences and is known by various acronyms such as FLASH, GRASS, Turbo-FLASH, Turbo-GRASS, SPGR, Turbo-SPGR and Ultrafast-SPGR. Such a fast gradient-echo sequence is described by Haase et al "FLASH Imaging: Rapid NMR Imaging Using Low Flip Angle Pulses," J. Magn. Res. 67:258-266; 1986, and is distinguished from the EPI sequence in that transverse magnetization is spoiled after acquisition of each phase encoding view and a new RF excitation pulse is applied before acquisition of the next view.
Regardless of the fast pulse sequence used, a scan is typically performed as a number of "shots" in which a set of NMR signals (e.g. 16) are acquired. For example, if 128 separate phase encoding views are acquired during a complete scan, one complete image data set can be acquired in eight shots (i.e. 128/16).
There are a number of well known NMR techniques for measuring the motion, or flow of spins within the region of interest. These include the "time-of-flight" method in which a bolus of spins is excited as it flows past a specific upstream location and the state of the resulting transverse magnetization is examined at a downstream location to determine the velocity of the bolus. This method has been used for many years to measure flow in pipes, and in more recent years it has been used to measure blood flow in human limbs. Examples of this method are disclosed in U.S. Pat. Nos. 3,559,044; 3,191,119; 3,419,793 and 4,777,957.
A second flow measurement technique is the inflow/outflow method in which the spins in a single, localized volume or slice are excited and the change in the resulting transverse magnetization is examined a short time later to measure the effects of excited spins that have flowed out of the volume or slice, and the effects of differently excited spins that have flowed into the volume or slice. Examples of this method are described in U.S. Pat. Nos. 4,574,239; 4,532,474 and 4,516,582.
A third technique for measuring motion flow relies upon the fact that an NMR signal produced by spins flowing through a magnetic field gradient experiences a phase shift which is proportional to velocity. This is referred to in the art as the "phase modulation" technique. For flow that has a roughly constant velocity during the measurement cycle the change in phase of the NMR signal is given as follows: EQU .DELTA..phi.=.gamma.M.sub.1 v
where M.sub.1 is the first moment of the magnetic field gradient, .gamma. is the gyromagnetic ratio and v is the velocity of the spins. By performing two complete scans with different magnetic field gradient first moments, an angiogram may be produced. Although there are a number of different procedures for producing such angiograms, in essence the signals from the two scans are subtracted such that stationary spins appear dark and moving spins appear light in the reconstructed image.
A number of techniques have been proposed for producing angiograms using fast NMR pulse sequences. For example, in a sequence described by D. N. Guilfoyle et al "Real Time Flow Measurements Using Echo Planar Imaging," Magnetic Resonance in Medicine 18, 1-8 (1991) an EPI pulse sequence is preceded by a preparatory sequence which destroys a component of the magnetization from flowing spins (M.sub.v sin.theta.) or else a component of the magnetization from flowing spins along with the magnetization from static spins (M.sub.v sin.theta.+M.sub.static) by applying a series of three RF pulses, a motion encoding gradient and a spoiler gradient. While this preparatory sequence suppresses magnetization associated with stationary spins, when applied to fast spin-echo or fast gradient-echo sequences stimulated echoes are induced by this preparatory sequence, and artifacts are produced in the image. In addition, the signals produced by stationary spins are not suppressed sufficiently by this sequence to provide good angiograms.